1. Field
The present invention relates to an optical transmitter and a control method therefor for adaptation to an optical transmission system. More specifically, the invention relates to control of an optical transmitter that generates and transmits a multiphase modulated signal.
2. Description of the Related Art
Research and studies for employing various types of modulation schemes for enhancing the capacity of transmission distance of optical transmission systems are popular. Modulation schemes under present research and studies include various types of modulation schemes, in addition to systems typically adapted to products, such as NRZ (non-return to zero) modulation schemes and RZ (return to zero) modulation schemes, for example, intensity modulation schemes, such as the CSRZ (carrier-suppressed return to zero) modulation scheme and the optical duobinary modulation scheme; and multiphase modulation schemes such as DPSK (differential phase shift keying) modulation schemes and DQPSK (differential quadrature phase shift keying) modulation schemes. Further, research and development are underway for optical transmission systems using these modulation schemes.
In such an optical transmission system, stabilizing techniques for stabilizing optical transmission signals are essential for components of an optical transmitter. Among such stabilizing techniques, there is, for example, an ABC (automatic bias circuit) that prevents a transmission signal from being deteriorated due to drift of a bias voltage of an LN modulator of a practical system using the NRZ modulation scheme operating with land or submarine communication.